1. Field of the Invention
The present disclosure generally relates to the field of semiconductor radiation detectors, and more particularly, to High-Purity Germanium (HPGe) detectors.
2. Description of the Related Art
With continuous development of the semiconductor manufacturing technology, a single crystal of HPGe with a large volume and good performance has been produced successfully such that HPGe detectors possess good energy resolution and relatively high detection efficiency. The HPGe detector may be used for, for example, measuring Gamma-rays, X-rays, detecting dark matter, radiation imaging techniques and the like. It has been applied widely in applications such as decay scheme research, internal conversion coefficient measurement, neutral reaction, short lifetime neutral measurement, activation analysis, neutral fuel research, mines detection, fine structure research of atomic nucleus, dark matter detection and the like.
Noise from the detector itself is one of the crucial factors restricting the energy resolution of the HPGe detector. The noise is mainly derived from the reverse leakage current and directly affects the energy resolution and sensitivity of the detector. In addition, detection of the reverse leakage current is an easy method for identifying process and determining the quality of products. Generally, a good HPGe detector should have the reserve leakage current less than 10 pA. However, it is not easy to reduce the reverse leakage current into this level; therefore, many researchers for detectors do a great deal of work on reduction of the reverse leakage current.
The reverse leakage current has three sources, i.e., bulk leakage current, diffusion leakage current and surface leakage current.
The bulk leakage current is current caused by electrons and holes generated by thermal activation. As the HPGe single crystal has a small energy band gap, in order to improve the resolution, HPGe detectors may be cooled into a temperature range from 70K to 100K in practice. Within the temperature range, the bulk leakage current may be negligible. The diffusion leakage current is current caused by doping contact. In case that minority of carriers has a very long lifetime, the diffusion current will not change as the reverse voltage varies, and thus may also be negligible. The surface leakage current is current related to a surface state of HPGe single crystal after packaging process and is generated at a surface layer. Typically, the surface leakage current is increased as the reverse voltage increases. Thus, the surface leakage current cannot be neglected, and is the main source of the leakage current of the detectors.
In the prior arts of reducing the surface leakage current of the detectors, the most direct and most important scheme is a surface cleaning method, in which a great deal of complex procedures are used to clean an ideal surface to keep the surface in a state of high resistance strictly. However, even if the strictest cleaning has been done in advance, as the crystal lattice terminates at the crystal surface, each germanium atom in the outmost layer of the surface will have one unmated electron, i.e., have an unsaturated bond. Due to these unsaturated bonds, the practical crystal surface often may be formed with a tiny oxidation film or may adsorb other atoms or molecules thereon, which makes the surface configuration complicated. In accordance with research, the clean surface without adsorbing any atoms or molecules will be remained in a short time even in high vacuum, and after a few hours, the surface will be formed with a single layer of atoms thereon. Besides the surface state caused by the above surface dangling bond, the surface states caused due to factors such as crystal defects or adsorption of various charged particles are also present in the surface. Factors such as the adsorbed various particles, mobile ions, fixed charges and traps in the oxidation layer of the surface may cause the reduction of the surface resistance of HPGe semiconductors, thus an electrical field is generated in the surface layer. These factors will adversely influence the features on the surface of the semiconductor significantly, in particular increase the surface leakage current, resulting in resolution reduction and dummy signals.
Another technology for reducing the surface leakage current of the detectors is surface passivation technology, i.e., a layer of passivation film having a function of blocking charged particles, such as amorphous germanium film, germanium oxide film or germanium dioxide film, is deposited on the clean HPGe semiconductor surface. The passivation technology has a positive effect on controlling the surface leakage current.